Lithium ion batteries are an important element part of the times of mobile devices and electric cars, and the need for increasing the capacity is constantly raised. In general, a lithium polymer ion battery has a basic structure in which positive electrodes and negative electrodes are stacked while overlapping with each other, and a separation membrane is inserted between the positive electrode and the negative electrode in order to prevent a short-circuit of the electrode. At this time, in order to prevent precipitation of lithium of the battery, the area of the negative electrode has to be wide enough so as to cover that of the positive electrode, and refers to an overhang. Since the capacity of the battery is determined by an area in which the negative and positive electrodes overlap as described above, the difference in area between the negative electrode and the positive electrode has to be minimized in order to increase the capacity in the same battery size, and as a result, when the overhang can be reduced, the capacity of the battery within the same area may be increased.
However, when an active material is coated, there is a problem in that an electrode active material is linearly coated by a coating process, and accordingly, there is a problem in that the overhang tolerance of the positive electrode and the negative electrode is increased for the reason, and the problems bring negative effects on the safety and capacity of the battery. In order to minimize the problems, end portions have to be made straight when coated with an electrode active material, but as the coating thickness becomes large due to high capacity of the battery, it is now becoming difficult to make end portions straight.
In order to solve the problems, for example, Korean Patent Application Laid-Open No. 10-2006-0010650 suggests a method for manufacturing a secondary battery by removing an active material in an area exceeding a reference mass using any one of laser, press, scraper, or ultrasonic wave, but since the method does not minimize the difference in area between active materials applied to the negative and positive electrodes, i.e. an overhang, there is a limitation in improving the capacity of a battery.